The present invention relates to a speed control circuit for the motor of an electric power tool, particularly a hand-held electric drill with an adjustable drilling speed. Various types of drilling machines are already known in which the fixed drilling speed is maintained independent of the load by means of an electronic regulating circuit. In a second switch position such circuits switch the motor over to the full operating speed. With such switches, a sudden variation or change in rotational speed is possible, perhaps resulting in a sudden reversal of the rotation force, occurring while the user is completely unaware and unprepared for any change, and consequently the machine could even be torn from his hands by the counter-rotational moment. Such a sudden reaction of the drilling machine to a change in direction could result in a high risk of damage or breakage to the mechanism, or even of injury to the user.